Because of the recent considerable advance in the medical technology, it has been recognized that the regeneration medical treatment including, for instance, the organ transplantation is important as a next-generation medical treatment for the purpose of treating the tissues, organs and/or parts or systems, which lose their shapes and/or functions. For example, there has been attempted cell transplantation while making use of a variety of stem cells as a therapy which can take the place of the heart transplant for the heart undergoing dysfunction, which suffers from a problem of the lack of donors.
There has recently been tried a tissue transplantation technique in which a three-dimensional myocardial tissue is, for instance, constructed in vitro and then the resulting three-dimensional myocardial tissue is transplanted into a patient. For instance, it has been known to produce various kinds of cell sheets and three-dimensional structures (see, for instance, Patent Document 1 specified below).
It has been believed that the key points of the regenerative medical techniques are the following three elements: cells, scaffolding materials for cells and humoral factors such as growth factors. Practically used as such scaffolding materials are three-dimensional porous bodies (sponge) mainly composed of collagen and poly(lactic acids) (see, for instance, Patent Document 2 specified below). There has been reported an example in which cartilage or the like can be regenerated through the use of these materials, but this technique suffers from such a problem that the technique can be applied only to injuries each extending over a small region. This is because such a technique cannot supply a large quantity of cells to the injured site although a large quantity of cells should be supplied thereto at the initial stage of the regeneration thereof; the materials cannot be sutured and cannot likewise satisfactorily be fixed to the damaged sites; and the materials do not have satisfactory compatibility, in particular, in the mechanical characteristics such as strength and elastic modulus.
On the other hand, there has been reported, as novel scaffolding materials, porous bodies mainly composed of silk proteins. However, the conventional porous bodies composed of silk proteins are produced according to a known manufacturing process mainly comprising the step of lyophilization and accordingly, the resulting porous bodies do not have satisfactory mechanical characteristics such as strength and elastic modulus and, for this reason, the applications thereof are quite limited as well. Contrary to this, there has also been known a method for the production of a hydrogel which shows sufficient mechanical strength (see, for instance, Patent Document 3 specified below). In this respect, it has been found that cells can undergo proliferation while they hold their differentiated states, within the hydrogels produced according to this technique, and it has been confirmed that a satisfactorily regenerated tissue is formed even at a very small number of initially inoculated or seeded cells (see, for instance, Non-patent Document 1 specified below). Accordingly, the hydrogel disclosed in Patent Document 3 may serve as an excellent scaffold material for the regeneration of a tissue, which has never been known formerly. However, these materials are not bio-absorbable ones unlike collagen or the like, while they are biodegradable ones and therefore, the applications of such a material has presently been limited even in the fields of, for instance, the regenerative medical techniques.
Moreover, when considering the more practical and clinical use of such a material, it would be desirable to achieve an effect of protecting the surface of an injured site during the term of the repair and regeneration of the damaged tissues; the secure suture and fixation of the material to the injured site; and the reduction of any mechanical load possibly applied to the injured site and it would likewise be desirable that the material makes it possible to regenerate tissues even when the damage thereof is severe and wide and the damaged sites lose a large number of normal cells and tissues. In addition, it is also desirable to protect the tissues opposite to the surface of injured sites during the term required for the repair and regeneration of the damaged tissues. However, there has not yet been developed presently any substrate (scaffolding materials) for supporting cells/tissues, which shows such an excellent function.